HP 24SII - Manual (Page 57)

If the computer is communicating with 7 bit word size with parity
enabled, the 8th or parity bit is masked from the Port Select Code
before it is examined, allowing you to send the Port Select Code with
odd or even parity.
Factory default is $SELECT.
In multiple printer applications, you may select printer ports either
anytime while printing or before the print job only. Mode A is a
general-purpose mode and allows the computer to select a printer
anytime while printing by sending a new port select sequence. Mode
B allows the computer to select the printer at the beginning of printing
only. In Mode B, the Model 24SII DES will recognize any port select
sequence in the first 16 characters of the print job only. After the first
16 characters of the print job have been received, the Input Inactivity
Timeout period must elapse before the Model 24SII DES will
recognize another port select sequence.
Computer-to-computer communication is not affected by the Port
Select Mode.
Factory default is Mode A: printer selection anytime while
printing.